MBG promotes natriuresis, but causes vasoconstriction and induces fibrosis via PKC&#61540;-dependent inhibition of Fli-1, a transcription factor and a negative regulator of collagen synthesis. We hypothesized that vascular smooth muscle cells (VSMC) from aged rats due to down-regulation of ANP/cGMP/PKG-dependent signaling would exhibit heightened sensitivity to the pro-fibrotic effect of MBG. In response to acute NaCl loading, aged (24-month old) Sprague-Dawley rats exhibited exaggerated MBG (5.40.4 vs. 1.90.2 pmol/hr;P<0.01) and pressor responses (29 vs. 15 mmHg;P<0.01), as well as greater inhibition of NKA in aorta as compared to young (3-month old) rats. Levels of vascular PKG1 and Fli-1 in aged rats were markedly reduced. In vitro, 1 nmol/L ANP prevented inhibition of vascular NKA by MBG in young, but not in the aged rats. In VSMC from young rats, 1 nmol/L MBG induced down-regulation of Fli-1 and a 1.5-fold increase in the levels of procollagen-1 and collagen-1, and 1 nmol/L ANP blocked this effect. In aged rats, levels of PKG-1 and Fli-1 in VSMC were markedly reduced vs. young rats, and 1 nmol/L MBG decreased Fli-1 (60%) and increased level of collagen-1 1.6-fold (P<0.01). In VSMC from aged rats, ANP failed to oppose the pro-fibrotic effect of MBG. Silencing of the PKG-1 gene in VSMC from young rats was associated with a marked increase in the sensitivity of VSMC to the pro-fibrotic effect of MBG;in this setting 1 nmol/L MBG increased levels of collagen-1 2.5-fold (P<0.01). These results demonstrate that the age-associated reduction in vascular PKG1 levels and resultant decline in cGMP signaling lead to the loss of ability of ANP to oppose MBG-induced inhibition of NKA and fibrosis. Silencing of PKG1 mimics effects of aging with respect to sensitivity of VSMC to pro-fibrotic action of cardiotonic steroids.